This invention relates to the protection of electrical components in computers and the like from electrical transients or surges caused, for example, by lightning and, more particularly, to a modular transient voltage surge suppressor (TVSS) having ultrafast fusing which prevents the plasma produced by a surge occurring in one part of a TVSS from causing subsequent damage in other parts of the TVSS.
TVSS devices are used with computers and similar equipment to protect them from electrical line surges caused by lightning or other transient conditions. TVSS devices are modular systems and may be connected in series or parallel as necessary. A particular advantage of the TVSS modules is they allow connections into electrical switchgear, or other electrical main service, without requiring external fusing. The modules have been tested and proved to withstand up to a 300 kA fault current at 600 V.
In operation, TVSS devices limit the peak voltages occurring in power line or electronic circuits since the components in computers and related equipment in these circuits can be damaged by a transient voltage condition resulting from a voltage surge. Strategic placement of TVSS devices mitigates the effect of voltage transients so they do not damage the system. Often, the device performs transient voltage suppression by functioning as a current diverter. In these instances, it presents a low impedance path for surge currents as compared to the impedance level of the protected circuits and equipment. Under normal operating conditions, a TVSS circuit draws very little current. And, as the voltage applied across the circuit increases above the design voltage level, the impedance of the TVSS circuit dramatically falls. This is highly desirable, because, for effective surge current diversion, very low impedance is required. For most effective TVSS operation, a non-linear voltage-current relationship is required for the suppression device. Solid-state TVSS devices use metal-oxide varistors (MOV's) or silicon avalanche diodes (SAD's) for this reason.
Despite the advantage of TVSS devices, they do have drawbacks. One of these is that energy delivered to a TVSS device is dissipated as heat. The more energy delivered to the device, the hotter the device becomes. If it gets too hot, as a result of a particularly large overvoltage transient, the device will fail. When it fails catastrophically, its SAD or MOV can partially vaporize. When this happens, the vapor can conduct electrical current. If sufficient voltage and current are available, a plasma forms which has several dangerous properties. It carries current and consequently bridges the current to any electrical metal contact it touches. The plasma may also bridge insulating spaces and short busses and wiring. This can result in damage to circuitry formerly protected or to other input equipment at the ac power input to the computer, and the surrounding equipment. The plasma releases heat which both destroys components and chars electrical insulation.
Failure of a TVSS device used on ac power lines then has several potential consequences. If the electronic components which the device is to protect includes other protection circuitry, there may be no resultant damage. On the other hand, if the line transient is so great that vaporization occurs, smoke or fire may result. At an extreme, an explosion so violent could occur that severe structural damage to the office, laboratory, or factory results. Personnel could be injured or killed.
In the design of TVSS equipment using MOV's or SAD's, the semiconductor device may have an associated fuse. Failure of a MOV or SAD is usually accompanied by a clearing (blowing) of its associated fuse as well. Catastrophic loss of either creates the vaporized metal condition described above and the attendant hazards of expanding plasma. Previous designs of the equipment have included some type of cover for one or the other to prevent dispersion of a metal vapor. This, however, is insufficient to afford the type of protection needed.